The continuous sheet thermoforming of synthetic plastic articles is a widely utilized industrial process. Thermoformers can take a variety of configurations, but each type includes the same basic stations. The plastic raw material is utilized in rolls or precut sheets. Common thermoformers include rotary machines, single stage devices, or as preferred for the purposes of the present invention, an in-line, continuous sheet thermoformer.
At the feed end of the in-line thermoforming process, a thin sheet of plastic is positioned at an in-feed or loading station. The loading station may receive the plastic sheet directly from an extruder, or from a rolled sheet of material. The thin plastic sheet is fed continuously, from the loading station to a heating station and then to a forming station. At the heating station the sheet is heated to the required forming temperature. In the forming station the sheet is deformed or molded into the desired shape. This forming is accomplished by direct compression or differential pressure forces, or a combination of these forces to fit the shape of the mold within the former.
For the thermoforming process, the sheet of thermoplastic material is heated until it becomes soft and pliable, but not fluid. The heated sheet is briefly held within the mold of the thermoforming press for forming.
Thermoforming systems with reciprocating platens suffer from slight off-cuts in the thermoformed articles produced, stemming from flexing related oscillations of the platen drive shaft. Flexing of the platen drive shaft translates to positional variation in the platen, and so in variation in the shape of the thermoformed article. Typically, the thermoformed article is manufactured to narrow tolerances.
The thickness of walls in a plastic cup, for example, must be thin to minimize the material required for the cup, but still must be thick enough to provide strength to the cup. A thousandths of an inch difference can result in a rejected cup, or in a cup failure. Consistency and precision are required in the manufacture of thermoformed articles, to maintain the desired precision at the high speeds required of modem thermoforming systems. Flex in the platen drive shaft is a limiting factor in attaining higher production rates.
The actuating linkages for thermoforming presses are typically complex and involve large forces that must be translated in an even and smooth mechanism in a high speed cycle. The inventors of the present invention observed that the standard, “sea saw” counterbalanced actuation of in-line thermoforming presses resulted in slight misalignment or registry errors that could not be compensated for, because these misalignments were inherent in the counterbalance system of platen drives.
A primary cause of these alignment errors are unwanted “moment arms” or extra-rotational forces on the platen drive shaft. These moment arms, if not offset or adequately reduced, alter the rotational motions of axial drive shafts that provided the force to raise and lower the platens in a reciprocating relationship. These misalignments are a larger problem at higher press operation speeds and under higher molding pressures, as required for thicker materials, cooler operating temperatures and faster press speeds.
U.S. Pat. No. 6,314,873 discloses a method for operating a thermoforming press that include the use of a “counter force assembly” 140, described therein as used to exert a counter force against the drive shaft of the thermoforming press and minimize deflection. The counter force assembly is shown in FIGS. 3, 4, 5A, 5B and 7, therein. The counteracting element of the '873 thermoforming press functions well and is a great advantage in reducing misalignments due to shaft flex over prior, in-line presses.
A new system of press operation is needed that minimized these alignment errors in the operation of a thermal press. The present invention addresses these operational alignment problems and provides a new system of thermoforming press operation. The aspects and advantages of the invention will become apparent from consideration of the following figures and description.